Although neutralizing antibodies (NAbs) are essential for protection after infection of many flaviviruses, relatively little is known about the key early events in innate immune cells that lead to effective neutralizing Ab responses. RIG-I like receptor (RLR) family members sense viral RNA and through the mitochondria associated adaptor protein MAVS, induce antiviral responses including the production of type I interferons (IFNs). Both MAVS and type I IFN pathways regulate Ab responses to viruses. In Project 3 we will determine the specific cell types that require MAVS and the type I IFN receptor (IFNAR) signaling to help develop the rapid first line Ab defense to WNV and later germinal center-dependent high affinity and long-lived NAb and memory B cell responses. Using a set of conditional knockout mice with either MAVS or IFNAR deleted in a cell type restricted manner, we will determine how MAVS or IFNAR function in dendritic cells (DCs), macrophages or B cells help develop effective humoral immunity to West Nile virus (WNV), an encephalitic flavivirus featured in this U19 program. As BAFF, a type I IFN-inducible cytokine, is essential for normal B cell development and responses to antigens, we also will define the role ofthe BAFF receptors, BAFFR and BCMA in humoral immune responses to WNV and determine which cells must produce BAFF for long-lived immunity to develop. Finally, as both monocytes and macrophages play important roles in both innate and adaptive responses to flaviviruses, we define how and when these cells contribute to flavivirus humoral immunity using mouse models in which subsets of these cells are deleted. Overall, this work will provide new insights in the interface between the innate immune and humoral responses, which may facilitate novel strategies for creating optimal humoral vaccine responses against flaviviruses.